1. Field of the Invention
The present invention relates to a rear-attachment lens system which is mounted between a photographic lens (hereinafter referred to as a master lens) and the body of a camera for the purpose of providing an increased composite focal length. More particularly, the present invention relates to a rear-attachment lens system of the three-unit-four-element type that employs an aspherical lens element at the position which is closest to the image plane.
2. Background of the Invention
Recent advances in lens design technology have been remarkable and appreciable improvements have been achieved in the performance of rear-attachment lenses which are intended to be mounted between the master lens and the body of a camera for the purpose of providing an increased composite focal length. A master lens is a typical camera lens that functions well by itself. The rear-attachment lens is inserted between the master lens and the camera body, thereby displacing the master lens away from the camera body, in order to change the composite focal length.
In the past, the design concept of rear-attachment lenses was based on the idea of using one rear-attachment lens for a limited scope of master lenses. However, commercial products available today have been developed in order to meet the requirement for using a single rear-attachment lens in association with a group of master lenses having a fairly wide range of focal lengths and F numbers.
Wide-angle and standard lenses greatly differ from telephoto and super-telephoto lenses with respect to the position and diameter of the exit pupil. Some of the rear-attachment lenses available today perform satisfactorily when they are used with wide-angle or normal master lenses which have an exit pupil at a position close to the image plane but if they are used with telephoto lenses, they experience astigmatism or a reduction in marginal illumination. Rear-attachment lenses suitable for use with telephoto master lenses have also been manufactured but since they are primarily designed to be combined with master lens of long focal lengths, they cannot be attached to wide-angle, normal or semi-telephoto lenses. In addition, these rear-attachment lenses are intended to be used with master lenses having fairly large F numbers and cannot be satisfactorily used with bright master lenses unless their aperture is stopped down to prevent the occurrence of large spherical aberrations.